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CS 1652

CS 1652. Jack Lange University of Pittsburgh. MAC Addresses and ARP. 32-bit IP address: network-layer address used to get datagram to destination IP subnet MAC (or LAN or physical or Ethernet) address:

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CS 1652

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  1. CS 1652 Jack Lange University of Pittsburgh

  2. MAC Addresses and ARP • 32-bit IP address: • network-layer address • used to get datagram to destination IP subnet • MAC (or LAN or physical or Ethernet) address: • function:get frame from one interface to another physically-connected interface (same network) • 48 bit MAC address (for most LANs) • burned in NIC ROM, also sometimes software settable 5: DataLink Layer

  3. LAN Addresses and ARP Each adapter on LAN has unique LAN address Broadcast address = FF-FF-FF-FF-FF-FF 1A-2F-BB-76-09-AD LAN (wired or wireless) = adapter 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 5: DataLink Layer

  4. LAN Address (more) • MAC address allocation administered by IEEE • Manufacturer buys portion of MAC address space (to assure uniqueness) • Analogy: • (a) MAC address: Your name • (b) IP address: Your postal address • MAC flat address ➜ portability • can move LAN card from one LAN to another • IP hierarchical address NOT portable • address depends on IP subnet to which node is attached 5: DataLink Layer

  5. Question: how to determine MAC address of B knowing B’s IP address? ARP: Address Resolution Protocol • Each IP node (host, router) on LAN has ARP table • ARP table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL> • TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) 137.196.7.78 1A-2F-BB-76-09-AD 137.196.7.23 137.196.7.14 LAN 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 137.196.7.88 5: DataLink Layer

  6. A wants to send datagram to B, and B’s MAC address not in A’s ARP table. Abroadcasts ARP query packet, containing B's IP address dest MAC address = FF-FF-FF-FF-FF-FF all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC address frame sent to A’s MAC address (unicast) A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state: information that times out (goes away) unless refreshed ARP is “plug-and-play”: nodes create their ARP tables without intervention from net administrator ARP protocol: Same LAN (network) 5: DataLink Layer

  7. 88-B2-2F-54-1A-0F 74-29-9C-E8-FF-55 E6-E9-00-17-BB-4B 222.222.222.221 1A-23-F9-CD-06-9B 111.111.111.111 222.222.222.222 222.222.222.220 111.111.111.110 R 111.111.111.112 49-BD-D2-C7-56-2A CC-49-DE-D0-AB-7D A B Addressing: routing to another LAN walkthrough: send datagram from A to B via R assume A knows B’s IP address • two ARP tables in router R, one for each IP network (LAN) 5: DataLink Layer

  8. 88-B2-2F-54-1A-0F 74-29-9C-E8-FF-55 E6-E9-00-17-BB-4B 222.222.222.221 1A-23-F9-CD-06-9B 111.111.111.111 222.222.222.222 222.222.222.220 B A 111.111.111.110 R 111.111.111.112 49-BD-D2-C7-56-2A CC-49-DE-D0-AB-7D This is a really important example – make sure you understand how this works! • A creates IP datagram with source A, destination B • A uses ARP to get R’s MAC address for 111.111.111.110 • A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram • A’s NIC sends frame • R’s NIC receives frame • R removes IP datagram from Ethernet frame, sees its destined to B • R uses ARP to get B’s MAC address • R creates frame containing A-to-B IP datagram sends to B 5: DataLink Layer

  9. Ethernet

  10. Ethernet “dominant” wired LAN technology: • cheap $0-10 for NIC • first widely used LAN technology • simpler, cheaper than token LANs and ATM • kept up with speed race: 10 Mbps – 100 Gbps Metcalfe’s Ethernet sketch 5: DataLink Layer

  11. Star topology • Bus topology popular through mid 90s • all nodes in same collision domain (can collide with each other) • Today: star topology prevails • active switch in center • each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other) switch bus: coaxial cable star 5: DataLink Layer

  12. Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: 8 bytes • 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 • used to synchronize receiver, sender clock rates 5: DataLink Layer

  13. Ethernet Frame Structure (more) • Addresses: 6 bytes • if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to network layer protocol • otherwise, adapter discards frame • Type: 2 bytes • indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk) • CRC: 4 bytes • checked at receiver, if error is detected, frame is dropped 5: DataLink Layer

  14. Ethernet: Unreliable, connectionless • connectionless: No handshaking between sending and receiving NICs • unreliable: receiving NIC doesn’t send acks or nacks to sending NIC • stream of datagrams passed to network layer can have gaps (missing datagrams) • gaps will be filled if app is using TCP • otherwise, app will see gaps • Ethernet’s MAC protocol: unslotted CSMA/CD 5: DataLink Layer

  15. 1. NIC receives datagram from network layer, creates frame 2. If NIC senses channel idle, starts frame transmission If NIC senses channel busy, waits until channel idle, then transmits 3. If NIC transmits entire frame without detecting another transmission, NIC is done with frame ! 4. If NIC detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, NIC enters exponential backoff: after mth collision, NIC chooses K at random from {0,1,2,…,2m-1}. NIC waits K·512 bit times, returns to Step 2 Ethernet CSMA/CD algorithm 5: DataLink Layer

  16. Jam Signal: make sure all other transmitters are aware of collision; 48 bits Bit time: 0.1 microsec for 10 Mbps Ethernet ;for K=1023, wait time is about 50 msec Exponential Backoff: Goal: adapt retransmission attempts to estimated current load heavy load: random wait will be longer first collision: choose K from {0,1}; delay is K· 512 bit transmission times after second collision: choose K from {0,1,2,3}… after ten collisions, choose K from {0,1,2,3,4,…,1023} Ethernet’s CSMA/CD (more) 5: DataLink Layer

  17. CSMA/CD efficiency • tprop = max prop delay between 2 nodes in LAN • ttrans = time to transmit max-size frame • efficiency goes to 1 • as tprop goes to 0 • better performance than ALOHA: and simple, cheap, decentralized! 5: DataLink Layer

  18. application transport network link physical fiber physical layer copper (twisted pair) physical layer 802.3 Ethernet Standards: Link & Physical Layers • many different Ethernet standards • common MAC protocol and frame format • different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10Gbps • different physical layer media: fiber, cable MAC protocol and frame format 100BASE-T2 100BASE-FX 100BASE-TX 100BASE-BX 100BASE-SX 100BASE-T4 5: DataLink Layer

  19. Manchester encoding • used in 10BaseT • each bit has a transition • allows clocks in sending and receiving nodes to synchronize to each other • no need for a centralized, global clock among nodes clock 5: DataLink Layer

  20. Link-layer Switches

  21. twisted pair hub Hubs … physical-layer (“dumb”) repeaters: • bits coming in one link go out all other links at same rate • all nodes connected to hub can collide with one another • no frame buffering • no CSMA/CD at hub: host NICs detect collisions 5: DataLink Layer

  22. Switch • Link-layer device: smarter than hubs, take active role • Store and forward Ethernet frames • examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment • Transparent • hosts are unaware of presence of switches • Plug-and-play, self-learning • switches do not need to be configured 5: DataLink Layer

  23. Switch: allows multiple simultaneous transmissions A • Hosts have dedicated, direct connection to switch • Switches buffer packets • Ethernet protocol used on each incoming link, but no collisions; full duplex • each link is its own collision domain • Switching:A-to-A’ and B-to-B’ simultaneously, without collisions • not possible with dumb hub C’ B 1 2 3 6 4 5 C B’ A’ switch with six interfaces (1,2,3,4,5,6) 5: DataLink Layer

  24. Switch Table A • Q: how does switch know that A’ reachable via interface 4, B’ reachable via interface 5? • A: each switch has a switch table, each entry: • (MAC address of host, interface to reach host, time stamp) • looks like a routing table! • Q: how are entries created, maintained in switch table? • something like a routing protocol? C’ B 1 2 3 6 4 5 C B’ A’ switch with six interfaces (1,2,3,4,5,6) 5: DataLink Layer

  25. Source: A Dest: A’ MAC addr interface TTL 60 1 A A A’ Switch: self-learning A • switchlearns which hosts can be reached through which interfaces • when frame received, switch “learns” location of sender: incoming LAN segment • records sender/location pair in switch table C’ B 1 2 3 6 4 5 C B’ A’ Switch table (initially empty) 5: DataLink Layer

  26. Source: A Dest: A’ A’ A MAC addr interface TTL 60 60 1 4 A A’ A A’ A A’ A A’ A A’ A A’ A A’ Self-learning, forwarding: example A • frame destination unknown: C’ B 1 2 3 flood 6 4 5 • destination A location known: C selective send B’ A’ Switch table (initially empty) 5: DataLink Layer

  27. S4 S3 S2 F I D H G E Interconnecting switches • switches can be connected together S1 A C B • Q: sending from A to G - how does S1 know to forward frame destined to F via S4 and S3? • A: self learning! (works exactly the same as in single-switch case!) 5: DataLink Layer

  28. Switches vs. Routers • both store-and-forward devices • routers: network layer devices (examine network layer headers) • switches are link layer devices • routers maintain routing tables, implement routing algorithms • switches maintain switch tables, implement filtering, learning algorithms 5: DataLink Layer

  29. Institutional network mail server to external network web server router IP subnet 5: DataLink Layer

  30. Switch: frame filtering/forwarding When frame received: 1. Record link associated with sending host 2. Look up switch table using MAC dest address 3. if entry found for destinationthen { if dest on segment from which frame arrivedthen drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived 5: DataLink Layer

  31. Self-learning multi-switch example Suppose C sends frame to I, I responds to C S4 1 S1 2 S3 S2 A F I D C B H G E 5: DataLink Layer

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